Extratropical Highlights –August 2023

 

1. Northern Hemisphere

The 500-hPa circulation during August featured predominantly above-average heights over western and southern North America, eastern Scandinavia to the Middle East, and eastern Russia, and weakly below-average heights over east-central North America and the North Pacific Ocean (Fig. E9). The main land-surface temperature signals include above-average temperatures for nearly all of Eurasia and North America (Fig. E1). The main precipitation signals include above-average rainfall totals in Europe, central Russia, and parts of North America, and below-average rainfall in southern North America (Fig. E3).

 

a. North America

The month of August featured a ridging pattern over the U.S. Southwest and Northwest, as well as western Canada and most of Alaska, and troughing over the U.S. New England states and Quebec (Fig. E9).  The ridging pattern contributed to strongly above-average temperatures in those regions with some areas reaching the 90th percentile of occurrences (Fig. E1).  The U.S. Southwest in particular observed large temperature anomalies (Fig. E1).  Despite the ridging pattern across the U.S. Northwest, above-average rainfall was observed in the region, as well as, in Southern California and the Inter-Mountain West, with some areas reaching the 90th percentile of occurrences (Figs. E3, E5, E6).  In New England, the Midwest, and Ohio Valley, above-average rainfall was also observed (Figs. E3, E5, E6).  However for the 3rd consecutive month, the Gulf Coast region has recorded below-average rainfall, with the month of August reaching the lowest 10th percentile of occurrences (Figs. E3, E5, E6).

 

b. Europe and Asia

The 500-hPa height pattern featured strongly above-average heights across northern Eurasia, the Middle East, and Spain (Fig. E9).  This pattern contributed to above-average temperatures across most of Eurasia with some regions seeing large anomalies and reaching the 90th percentile of occurrences (Fig. E1).  Many areas across Europe continue to set new temperature-related records as the hottest summer on record comes to a close.  Precipitation totals were above normal for parts of Scandinavia, eastern Europe, and central Russia with some areas recording rainfall in the highest 90th percentile of occurrences (Fig. E3, E4). Western and eastern Russia observed below-average rainfall with some areas reaching the lowest 10th percentile of occurrences (Fig. E3).

 

c. West African monsoon

The West African monsoon extends from June through September, with a peak during July-September. During August 2023, the west African monsoon system was below-average with rainfall totals reaching the lowest 10th percentile of occurrences in some areas (Fig. E3) and the lowest 20th percentile overall for the Sahel region (see Sahel region, Fig. E4).

 

2. Southern Hemisphere

Above-average heights were observed across much of Australia, Chile, the Ross Sea, the South Indian Ocean, and Queen Maud Land of Antarctica (Fig. E15). The main land-surface temperature signals for August feature above-average temperatures for much of South America, Australia, and Africa, with many of these areas reaching the highest 90th percentile of occurrences (Fig. E3).  The main land-surface precipitation signals include below-average rainfall totals for Columbia, Venezuela, northern Brazil, the Sahel and central regions of Africa, and along the Great Dividing Range of Australia's eastern half, with some of these areas reaching the lowest 10th percentile of occurrences (Fig. E3).  The Antarctic ozone hole typically develops during August and reaches peak size in September. The ozone hole then gradually decreases during October and November, and dissipates on average in early December (Fig. S8). By the end of August 2023, the size of the ozone hole approached approximately 21 million square kilometers, which is larger than the largest of ozone holes observed during the 2012-2021 period (Fig. S8).  Typically the size of the ozone hole is associated with an above-average polar vortex area and above-average polar stratospheric cloud coverage.  However for August, those variables were measured at near normal values (Fig. S8), thus it is speculated that the Hunga Tonga eruption last year in the South Pacific Ocean added higher water vapor levels to the stratosphere and the added water vapor is possibly contributing to the significantly larger than average ozone hole in August 2023.